From ce7068d3783495a4b395660f89a107f83a9eea3f Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Fri, 25 Mar 2005 00:02:41 +0000
Subject: [PATCH] Make the spliceFrom case where one graph is completely empty
 be constant time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@20825 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/DataStructure/DataStructure.cpp | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/lib/Analysis/DataStructure/DataStructure.cpp b/lib/Analysis/DataStructure/DataStructure.cpp
index 1133743b5d3..6460287ac71 100644
--- a/lib/Analysis/DataStructure/DataStructure.cpp
+++ b/lib/Analysis/DataStructure/DataStructure.cpp
@@ -1321,8 +1321,12 @@ void DSGraph::spliceFrom(DSGraph &RHS) {
   AuxFunctionCalls.splice(AuxFunctionCalls.end(), RHS.AuxFunctionCalls);
 
   // Take all of the return nodes.
-  ReturnNodes.insert(RHS.ReturnNodes.begin(), RHS.ReturnNodes.end());
-  RHS.ReturnNodes.clear();
+  if (ReturnNodes.empty()) {
+    ReturnNodes.swap(RHS.ReturnNodes);
+  } else {
+    ReturnNodes.insert(RHS.ReturnNodes.begin(), RHS.ReturnNodes.end());
+    RHS.ReturnNodes.clear();
+  }
 
   // Merge the scalar map in.
   ScalarMap.spliceFrom(RHS.ScalarMap);